Recently, for optical members (e.g. prism sheets, overcoating agents, hard coating agents, antireflection films or optical films, optical sheets, optical fibers, optical waveguides, holograms, films for liquid crystals, films for organic EL, various optical lenses and the like, which are used for liquid crystal display devices), various transparent resins (polycarbonate, polyester and the like) are frequently used in view of advantages such as favorable molding processability and high productivity or weight reduction and thickness reduction. From the viewpoint of thickness reduction of the optical members, the resins are required to have a further high refractive index.
Among these various transparent resins, resins in which a bisphenol having a fluorene skeleton is used as a raw material monomer have attracted attention as transparent resins for optical members in recent years because they have a high refractive index, and are excellent in heat resistance etc. Among them, resins produced from a bisphenol having a structure represented by the following formula (5):
[Chemical Formula 1]
and having a phenyl group as a substituent are known to have a particularly high refractive index and exhibit excellent heat resistance (e.g. PTDs 1 and 2).
However, the bisphenol represented by the formula (5) has a very high melting point of about 270° C., and thus has the problem that the bisphenol is difficult to handle as a resin raw material. In particular, in the case of a melting polymerization method or the like in which a resin is produced without using a solvent, it is normally necessary to melt a raw material once with the reaction temperature set to a temperature higher than or equal to the melting point of the raw material at the time of carrying out a polymerization reaction, and therefore a bisphenol having a high melting point is used as a raw material, there is the problem that the reaction temperature must be increased, and thus the resin is easily colored during the reaction.
In addition, as resins that can be used for the above-mentioned optical members, for example, polyarylate resins, resins obtained by curing (polymerizing) a curable composition containing a (meth)acrylate compound, and the like are known.
The polyarylate resin is a thermoplastic wholly aromatic polyester having a constituent unit derived from a bisphenol and a constituent unit derived from an aromatic dicarboxylic acid, and has characteristics such as relatively high heat resistance, high transparency and a high refractive index.
As a polyarylate resin having a high refractive index among the above-mentioned polyarylate resins, for example, PTD 3 discloses a polyarylate resin having a constituent unit derived from 9,9-bis(hydroxy-fused polycyclic aryl)fluorene, and a constituent unit derived from an aromatic dicarboxylic acid, and the polyarylate resin has a very high refractive index of 1.71 to 1.74. However, the polyarylate resin has a very high glass transition temperature of 312 to 340° C., and is difficult to use as a hot melt molding material.
Further, in the literature, a polyarylate resin having a constituent unit derived from 9,9-bis(4-hydroxy-3-methylphenyl)fluorene is shown as a comparative example, and the polyarylate resin has a refractive index of 1.64, which is comparable to that of a general optical transparent resin, and has a relatively low glass transition temperature of 289° C., but as a result of examining the melt fluidity of the polyarylate resin, the inventors of the present application have found that the polyarylate resin has almost no melt fluidity even at 360° C., and is difficult to use as a hot melt molding material as in the case of the above-mentioned polyarylate resin.
In addition, it is known that among (meth)acrylate compounds, a difunctional (meth)acrylate compound derived from a bisphenol having a fluorene skeleton can form a resin exhibiting a high refractive index (e.g. PTD 4).
However, many of difunctional (meth)acrylate compounds derived from a bisphenol having a fluorene skeleton have a very high viscosity, or are in the form of a solid (powder), and therefore in production of a cured product, the viscosity should be reduced by forming the difunctional (meth)acrylate compound into a curable composition using a diluent such as an organic solvent or a monofunctional (meth)acrylate. However, many of difunctional (meth)acrylate compounds derived from a bisphenol having a fluorene skeleton have poor compatibility with the diluent, or a low solubility in the diluent, and therefore it is known that there arises the problem that preparation of a curable composition containing the (meth)acrylate compound in a high concentration is difficult, or the problem that a high refractive index to be developed by the (meth)acrylate compound is not sufficiently exhibited due to deflection of the refractive index caused by the diluent (e.g. PTD 5). In PTD 5, a cationically polymerizable liquid compound having an aromatic structure is added for solving the above-mentioned problem, but when a cationically polymerizable liquid compound is added, the refractive index tends to decrease as compared to a case where the cationically polymerizable liquid compound is not added.
In addition, a (meth)acrylate compound produced by ethylene-oxidating or propylene-oxidating the hydroxyl group of a bisphenol having a fluorene skeleton, and then (meth)acrylating the resultant group is known to be liquefied or have a reduced viscosity, or have improved compatibility or solubility with a diluent as compared to a (meth)acrylate compound produced without passing through such a step (e.g. PTD 6). However, even with the compound produced in this manner, the above-mentioned problem is not sufficiently solved in many cases, and further, there is the problem that the refractive index decreases as compared to a bisphenol compound produced without passing through the ethylene-oxidation or propylene-oxidation step, or the problem that the production cost and the number of steps for the resulting (meth)acrylate compound increase as the ethylene-oxidation or propylene-oxidation step is carried out.
On the other hand, epoxy resins generally form cured products excellent in mechanical properties, water resistance, chemical resistance, heat resistance, electrical properties and the like when cured with various curing agents. Thus, epoxy resins are used in a wide range of fields such as those of adhesives, coating materials, laminated sheets, molding materials, and casting materials. Among these epoxy resins, epoxy resins having a fluorene skeleton have a high refractive index, and are excellent in heat resistance etc., and these epoxy resins are being under extensive research and development for use in new fields such as those of sealing materials for semiconductor light emitting elements such as optical lenses and light emitting diodes (LEDs) in addition to the above-mentioned general fields where epoxy resins are used (e.g. PTD 7).
In particular, among epoxy resins having a fluorene skeleton, epoxy acrylate resins obtained by preparing an epoxy resin by epoxidizing a bisphenol represented by the formula (5), which has a phenyl group as a substituent, and further reacting the epoxy resin with acrylic acid are known to be used as raw materials for color filters that are suitably used for color liquid crystal displays etc. (e.g. PTD 8), or as films excellent in gas barrier property (e.g. PTD 9).
Thus, the epoxy resin having a fluorene skeleton is used not only as a sealing material obtained by curing the resin itself but also as a raw material for a resin obtained by reaction with other compounds such as acrylic acid and having a new structure, but there is the problem that the epoxy resin having a fluorene skeleton is poor in solubility in an organic solvent, and conditions for reaction with other compounds are limited.